1. Field of the Disclosure
This disclosure generally relates to an optical navigation system and, more particularly, to an optical navigation system and detection method thereof capable of detecting ambient light and liftoff.
2. Description of the Related Art
The conventional optical navigation device, such as an optical mouse, generally has a light source, an image sensor and a processing unit. When a user operates the optical mouse on a working surface, the light source illuminates the working surface and the image sensor receives light reflected from the working surface. The processing unit of the optical mouse calculates a movement value corresponding to the user's operation according to the images successively captured by the image sensor and converts the movement value to an electric signal. A host then relatively controls a cursor movement according to the electric signal.
However, the optical mouse possibly leaves the working surface due to the user's operation when being operated. If the optical mouse still captures inexact images of the working surface, the processing unit calculates an incorrect movement value thereby causing misoperation such as the cursor jitter.
In order to allow the optical navigation system to stop calculating movement values and reduce power consumption when the optical navigation leaves the working surface, U.S. Pat. No. 8,044,936, entitled “Optical navigation device and method of operating the same” discloses an optical navigation device that detects whether the optical navigation device leaves a working surface to prevent misoperation and reduce unnecessary power consumption. FIG. 1 is a timing diagram provided by the prior art and the timing control thereof is composed of an effective frame Fa and a dark frame Fd arranged alternatively, wherein the effective frame Fa includes a first period P1 and a second period P2, and the dark frame Fd includes a third period P3 and a fourth period P4. The first period P1 is a bright exposure period (i.e. exposing a photosensitive unit when the light source is being turned on). The second period P2 is a bright image readout period. The third period P3 is the dark exposure period (i.e. exposing the photosensitive unit when the light source is turned off) and displacement calculation period. The fourth period P4 is a dark image readout period. The prior art is to insert the dark frame Fd right after the effective frame Fa to obtain dark image intensity, and then compare the dark image intensity with bright image intensity of the effective frame Fa so as to detect whether the optical navigation device is lifted off.
However, an optical navigation system calculates displacement according to successive bright images. The method which detects liftoff through inserting a dark image between two bright images according to the prior art causes an extension of a time gap between the two bright images thereby decreasing the frequency (or bandwidth). In a high speed optical navigation system (e.g. a gaming mouse), a relatively higher effective frame rate is needed for supporting the liftoff detection and maintaining an identical bandwidth simultaneously. Therefore, if it is desired to maintain an identical tracking speed by using the conventional method mentioned above, the power consumption of the optical navigation system is increased.